1. Field of the Invention
The present invention relates to an optical pickup which performs recording and/or reproducing of information signals with respect to information recording media, such as optical discs, and also related to an optical device constituting the optical pickup.
2. Description of the Related Art
A variety of optical discs have been hitherto used as consumer information recording media. For these optical discs, there exist so-called “CD” (Compact Disc), “DVD” (Digital Versatile Disc, etc. in widespread use. Recently, not only playback-only standards but various recordable standards (e.g. “CD-R”, “CD-RW”, “DVD-RAM”, “DVD-R”, “DVD-RW”, “+R”, “+RW”, etc.) have been in widespread use rapidly. In addition, there are also proposed “BD” (Blu-ray Disc) and “HD-DVD” as optical discs corresponding to high-definition video signals.
In the optical pickup, which is essential to a recording/reproducing device (optical-disc system) for recording and/or reproducing information signals to and from these optical discs, the function of receiving and emitting light is fundamental. Nevertheless, due to the operations of focus-servo (focusing adjustment) and tracking-servo (track-following adjustment), which are physically-basic actions in the recording/reproducing device, optical and electrical functions for detecting a focus-error signal and a tracking-error signal used for these servo operations are essential to the optical pickup.
In order to allow such an optical pickup to correspond to optical discs based on a plurality of standards mentioned above, it is necessary for the optical pickup to have the capability of detecting a plurality of error signals best for the optical discs based on respective standards. Under such a situation, there is required an optical pickup having a rational optical system enabling both simple structure and improved accuracy, in the market. In fact, there is a development race of such an optical pickup in this field.
An optical device where a laser beam light source, a light-receiving element and a hologram element having a divergent function of outward and backward optical paths and a lens function of error generation are integrated with each other, has contributed much to simplification of the optical pickup in constitution and therefore, there have been proposed new technologies in this field. In addition, a two-wavelength pickup and a two-wavelength integrated device have been proposed in order to accomplish compatibility between “CD” system and “DVD” system. Japanese Patent Publication Laid-open No. 2000-123376 has disclosed an optical pickup having an integrated light receiving/emitting function of two wavelengths.
In an optical pickup to meet a recordable standard, an optical system incorporating a Can laser unit advantageous to heat radiation or a polarization beam splitter becomes mainstream. For the technology to simultaneously dissolve contradictory propositions in the optical pickup meeting the recordable standard, International Patent Publication Laid-open No. WO2004/003901A1 has disclosed a constitution that embodies both a function of detecting two wavelengths for “CD” and “DVD” and a function of emitting light for “CD” in the form of an integrated optical device, while the Can laser unit for “DVD” is independent of the optical device.
Furthermore, Japanese Patent Publication Laid-open No. 2007-200471 has disclosed the structure of an integrated device focusing attention on simplification of a light-receiving element (or photo detector IC: PDIC). In this integrated device, it is possible to not only accomplish both simplification in circuit size and reduction in manufacturing cost but simplify the light-receiving element and the circuit while maintaining the function of detecting two wavelengths.